1. Field of the Invention
This invention concerns a thulium doped fluorozirconate host based optical amplifier and laser, and more particularly to an upconversion pumped thulium fluorozirconate amplifier and laser operating at 790-820 nm.
2. Description of the Related Art
A broad-bandwidth fiber laser amplifier at wavelengths in the vicinity of 810 nm is useful for a number of applications including short-haul telecommunications (the so-called first telecommunications window), direct in-fiber generation of high power cw and pulsed laser light, and chirped pulse amplification of ultrashort pulses. To date, no suitable high-power amplifiers in this wavelength range have been demonstrated. To be fully satisfactory, such an amplifier should be high gain (up to 30 dB), high saturation output power (in excess of 20 dBm [100 mW]), and compatible with short-haul fiber optic communications systems.
Configured as a laser, such a system should yield very high powers (in excess of 1 W) directly from a fiber, with high power conversion efficiency (in excess of 35%). Such a laser would be suitable for generation of high peak power pulses via Q-switching or mode locking. It would be desirable to have such a laser and amplifier pumped at a wavelength where efficient, high power sources are readily available so that these high gains and output powers can be obtained with high efficiency.
While no known device suits all these needs, it is noted that there are sources and amplifiers available at this wavelength. Certain solid state lasers, e.g., the titanium doped sapphire (Ti:Al.sub.2 O.sub.3) and chromium doped colquiriites (Cr:LiSAF, Cr:LiCAF), provide laser light tunable over this wavelength range at powers of the order of 1 W. However, these are bulk-optic systems, and so are not directly fiber-compatible. The Ti:Al.sub.2 O.sub.3 amplifier has been extensively developed, however performance of such systems is limited by the relatively short excited state lifetime of the gain, and they must be pumped by, for example, a large frame Ar.sup.+ laser requiring high voltage, high current, and chilled water service for operation. Semiconductor diode lasers based on AlGaAs have been developed with output powers greater than 1 W in the wavelength regime of interest. However, high powers are typically achieved with array or broad area devices, which cannot be efficiently coupled to single mode fibers.
The following publications exemplify the art:
Thulium doped fluorozirconate fibers have been used to obtain amplification and lasing in the 800-820 nm band when pumped at 780 nm [J. N. Carter et al., Electron. Lett. 26, 1759-61 (1990); J. N. Carter, et al., J. Lightwave Technol. 9, 1548 (1991); R. G. Smart, et al., Electron. Lett. 28, 58 (1992)]. However, sufficiently high power single mode sources are not available at the pump wavelength to obtain high signal powers (e.g., 1 W and higher).
Operation at 1470 nm has been demonstrated with pumping at 1064 nm [T. Komukai, et al., Electron. Lett. 29, 110 (1993); Y. Miyajima, et al., Electron. Lett. 29, 660 (1993)]. However, this signal wavelength falls outside of the first telecommunications window. The pumping scheme described by the authors involves a two-step absorption and does not necessarily involve an avalanche absorption process. As will be described below, the inventors believe that an avalanche absorption process is what permits laser output between about 790 nm and about 830 nm when the invention described below is pumped as described below.
Lasing at wavelengths between 478 and 483 nm has been obtained in Tm:ZBLAN fiber when pumped by multiline Nd:YAG laser operating simultaneously at wavelengths of 1112, 1116, and 1123 nm [Grubb, et al., Electron. Lett. 28, 1243 (1992)].